Conventionally, a plastic molded article and a paper/pulp molded article are the mainstream of a disposable molded article that is disposed after the use. This is because in most cases, raw materials of the disposable molded article should be durable and strong and at the same time readily formable depending on usage of the molded article.
However, the plastic molded article and the paper/pulp molded article both have difficulties to be used as the disposable molded article, as shown below.
First, when incinerated, the plastic molded article damages an incinerator because of the generation of extremely high heat, or additionally produces environmental pollutants such as dioxin. Also, when the plastic molded article is buried for reclamation, it is impossible to bury the molded article where once the same is buried, since plastics are rarely decomposed naturally. Moreover, due to a recent increase of the amount of waste, it has become difficult to secure new dumping sites year after year. Furthermore, continuous environmental pollution for a long period of time could be caused, since the plastic molded article cannot be decomposed easily.
Also, reserves of fossil fuels such as oil, which are raw materials of plastics, are decreasing year after year, so the plastic molded article could become more expensive in the future.
Meanwhile, the paper/pulp molded article is superior to the plastic molded article in that it can easily be disposed of by incineration and can be decomposed naturally. However, trees, of which paper and pulps are made, grow slowly and thus the mass consumption of paper and pulps substantially reduce forest resources. The reduction of forest resources brings about not only heavy destruction of an environment of the area but also a great impairment of an ability of forests to absorb CO2, and a spun for global warming because of the increase of CO2 from a broad perspective.
So, to solve the aforementioned problems, particularly from an environmental perspective, the disposal method of the molded article has recently shifted from dumping to recycling.
However, as for the recycling, for instance, when a food container, one of the main use of the disposable molded articles, is recycled, residues in the container such as remaining foods and seasonings have to be removed. This is required to avoid a contamination of impurities in the material to be recycled.
Usually the removal of the residues is washed in water, so this induces another pollution such as an increase of the amount of polluted water and subsequent water pollution in rivers and the sea. Also, the recycling requires high cost, because the removal of the residues itself lowers the efficiency of recycling as it needs a lot of time and efforts, and the system of recycling has not been really socially established at the moment.
So, what has become a focus of attention is a recently-developed biodegradation disposal method for the molded article by using microbe, as a new disposal method of the molded article being different from the recycling. This disposal method can avoid the problems above, since in this case the molded article is mainly made of various biodegradable plastics or natural high polymers such as starch.
Especially, in the biodegradation disposal method above, a method to utilize the natural high polymers such as starch and protein particularly draws attentions in terms of its practicality. This is because the various biodegradable plastics have a problem that despite having a fine quality almost comparable to conventional plastics (non-degradable or degradable-retardant), practically they cannot be decomposed quickly enough.
For instance, when the thickness of a molded article made of the biodegradable plastic is heavy, it takes a very long time until the molded article is completely decomposed, so practically it is not possible to produce a molded article with enough volume. Also, when the molded article made of the biodegradable plastic is used practically as a disposable food container, composting the molded article together with food residues is the least harmful disposal method for the environment. However, actually it is difficult to compost them together since the biodegradable plastic above is only decomposed much slower than the food residues. Furthermore, it is also difficult to crash the molded article to hasten the decomposition of the biodegradable plastic, because normally the molded article cannot be crushed easily when it has a certain thickness and strength. Thus it is almost impossible to compost the molded article made of the biodegradable plastic.
Whereas starch and protein, etc. are positively evaluated as the materials because of advantages such as:                with fine biodegradability, decomposition is quite easy even if the volume is large;        the resource can be acquired easily on account of an availability of a vegetable starch that is mass-produced by agriculture; and        a molded article with adequate thickness and thermal insulation can be acquired, since the molded article is usually an expanded molded article.        
(1) Japanese Laid-Open Patent Application. No. 5-320401/1993 (Tokukaihei 5-320401; published on Dec. 3, 1993, (2) Japanese Laid-Open Patent Application No. 7-224173/1995 (Tokukaihei 7-224173; published on Aug. 22, 1995), (3) Japanese Laid-Open Patent Application No. 7-10148/1995 (Tokukaihei 7-10148; published on Jan. 13, 1995), (4) Japanese Laid-Open Patent Application No. 2000-142783 (Tokukai 2000-142783; published on May 23, 2000), and (5) Japanese Laid-Open Patent Application No. 7-97545/1995 (Tokukaihei 7-97545 published on Apr. 11, 1995) disclose biodegradation disposal technologies using starch, protein, etc.
First, a molded article derived from the technology (1) or (2) have the advantages that it has better decomposability than a molded article mainly made of the biodegradable plastic and also superior to those derived from paper/pulps in its diversity of the molded shape, since natural starch is mainly used as the material. However, at the same time the molded article derived from the technology (1) or (2) has the disadvantages that it can be used only for limited purposes and is required to barrier moisture, due to its poor water and humidity resistance.
Second, a molded article derived from the technology (3) or (4) is mainly made of starch or similar polysaccharide, and to enhance its water resistance, a natural resin (dammer resin, shellac resin, etc.) is painted on the surface of the molded article to form a water-resistant coating.
However, the surface of the molded article (including expanded molded article) mainly made of starch cannot be completely smoothed, and generation of small irregularities cannot be avoided. Thus small pinholes are likely to be formed on the surface corresponding to the irregularities of the water-resistant coating if the resin is simply painted, so it could be possible to render the molded article water repellent but difficult to make the same complete water proof. Particularly, if the molded article is required to be moisture-resistant, moisture is likely to be absorbed from the pinholes on the water-resistant coating, and the molded article becomes apt to be disfigured.
Furthermore, the dammer resin, the shellac resin, etc. must be dissolved in an organic solvent such as alcohol, etc., when applied to the surface. So this introduces problems in terms of a manufacturing facility. For instance, when the organic solvent is removed after the paint, large-scale equipment is required to prevent diffusion of the organic solvent in the air that causes air and environment pollution.
Now, on a surface of a molded article derived from the technology (5) that is made of, as in the cases of the molded articles of the technologies (3) and (4), poorly water-resistant biodegradable material such as starch, a biodegradable coating agent composed of aliphatic polyester being dissolved in halogenated hydrocarbon is painted. In this case, using a dip method (dip coating method) for actual coating of the surface, an adequately water-resistant coating can be formed even on a complicatedly-shaped molded article.
However, in this method, it is required to remove the halogenated hydrocarbon used to dissolve the coating agent, and as in the case of the technologies (3) and (4), problems such as a requirement of equipment to prevent diffusion of halogenated hydrocarbon arise. Many halogenated hydrocarbons are often harmful for a human body and the environment, and moreover the halogenated hydrocarbon that is concretely mentioned in the technology (5) contains CFC so that it should be released to the air as little as possible. On this account, a large-scale hermetic room and a reclaiming device are required as the equipment above.
In addition to the technologies introduced above, there is a technology in which wax or hydrophobic protein, prepared as a solution to be applied, is painted on the surface of the molded article. Generally, speaking, it is difficult to paint a water-resistant coating on the surface of the molded article evenly and entirely, while the coating on a flat molded article such as a flat plate is relatively easy. However, small irregularities are likely to be formed on the surface of the molded article mainly made of starch as described above and obstruct the formation of an uniform film, and furthermore, the molded article or a painting device has to be rotated when the molded article is roughly circular in cross section, for instance formed like a cup or a bowl. Therefore the painting becomes more difficult.
Besides, even if the coating agent can be painted evenly and entirely by using the dip method, the painted agent runs down before it solidifies and becomes the coating, and an unevenness is likely to show up on the coating.
The wax has a problem of poor heat resistance due to its relatively low melting point. In the meantime, although the hydrophobic protein has better heat resistance and does not need the organic solvent, the molded article absorbs water and is softened/disshaped in the painting process owing to a frequent use of aqueous solvents.
So, a technology that has already been proposed is to laminate a water-resistant coating instead of painting thereof. More specifically, such examples include (6) Japanese Laid-Open Patent Application No. 11-171238/1999 (Tokukaihei 11-171238; published on Jun. 29, 1999), (7) Japanese Laid-Open Patent Application No. 5-278738/1993 (Tokukaihei 5-278738; published on Oct. 26, 1993), (8) Japanese Laid-Open Patent Application No. 5-294332/1993 (Tokukaihei 5-294332; published on Nov. 9, 1993).
A container of the technology (6), made by a pulp molding method instead of molding starch, is covered by a water-impermeable or non-absorbing protective coat. This method has the advantage that conventional plastic coating method can be applied almost without any change. However, at the same time the method has problems such as:                the biodegradation of the pulp-molding takes place slowly since it is made of fiber so that the molded article cannot be disposed together with remaining foods, etc.; and only limited types of the molded article can be produced because it is difficult to make the molded article thicker, and also the molded article is not suitable for a deep drawing.        
Meanwhile, a thin film made of biodegradable plastic is formed on a surface of a biodegradable container of the technologies (7) or (8) made of either one of natural polysaccharide or protein, or either of the two materials that are chemically modified but still biodegradable.
In this technology, while the biodegradable plastic is provided as the thin water-resistant coating, the container itself is made of natural polysaccharide, protein, etc. with enough thickness. On this account, the container is sufficiently water-resistant as well as biodegradable. Thus it can be said that this technology is particularly promising among the disposal technologies by dint of biodegradation using starch, protein, etc.
However, the technology (7) is an arrangement that the biodegradable plastic thin film simply covers the main body of the biodegradable container, and a concrete arrangement of the biodegradable container is hardly mentioned.
For instance, when the main body of the biodegradable container is mainly made of polysaccharide or protein, the main body's strength should be cared of, but the technology (7) does not mention the strength at all. Also, the technology does not explain how the biodegradable plastic thin film is actually formed, such as by painting, by attaching preformed film, etc., for instance.
Moreover, the technology (7) does not stipulate the coating state of the biodegradable plastic thin film with respect to the main body of the biodegradable container at all. The biodegradable plastic thin film covers the main body of the biodegradable container mainly made of polysaccharide or protein, to improve the main body's water resistance. But the technology (7) does not mention anything except that the main is covered, so there is no statement about how it is covered.
Even if the biodegradable container is made as disposable one, still the container should have a stability and durability as a one-way container. So the biodegradable plastic thin film should not fall off from the main body of the biodegradable container, and thus the state of coating on the main body of the container is an important factor, but no mention with respect to this can be found in (7).
Furthermore, as already described, it is difficult to use biodegradable plastics as a thick molded article due to its slow biodegradation, so the speed of the biodegradation also greatly depends on not only the thickness of the molded article but also a total amount of biodegradable plastics contained in the molded article. In relation to this, the technology (7) only describes that an effectiveness of the biodegradation is improved if the main body of the biodegradable container is expanded, and there are not comments on a relationship between a degree of the expansion and the biodegradation, and a balance between the biodegradation of the biodegradable plastic and that of the main body of the biodegradable container. As a result, it is not possible to manage the biodegradation of the whole container favorably.
In the meantime, the technology (8) can be assumed to correspond to one of the manufacturing technologies of the biodegradable container disclosed by (7). In this technology, a thermoplastic is dissolved in a solvent and painted on the surface of the main body of the biodegradable container. Then after the solvent is dried and volatilized, another coating thin film made of a thermoplastic is laminated and bonded by thermocompression. That is to say, the technology (8) discloses that thermoplastic is used as an adhesive to bond the coating thin film (equivalent to the biodegradable plastic thin film) securely.
Now, as described in relation to technologies (3) to (5), when the thermoplastic dissolved in the solvent is used, problems such as a requirement of equipment to prevent diffusion of the solvent arise. Moreover, an embodiment of (8) uses chloroform as the solvent and this substance should be scattered in the air as little as possible, thus as in the case of (5), a large-scale hermetic room and reclaiming device are required as the equipment above.
Also, the manufacturing technology of (8) acquires the main body of the biodegradable container by press-molding a sheet made of polysaccharide or protein that is formed in advance in a metal mold. Thus it is impossible to mold molded articles such as a container with deep drawing shape like a cup, molded articles having irregular thickness like a food tray with partitions and a wrapping tray, and molded articles having complex shape like cushioning material for wrapping.
Also, in a conventional art relating to a biodegradable container derived from starch, sufficient strength is acquired. However, “strength” generally means strength under a dry condition and normal atmosphere such as “piercing strength”. As mentioned below, according to the present inventors, there is no relation between strength under a dry condition and normal atmosphere and strength under moisture absorption (moisture absorption strength). Therefore, even if the biodegradable container derived from starch has sufficient “strength” what is generally called, it does not have sufficient moisture absorption strength and it may soften or disshaped due to moisture absorption during a long-term storage under high humidity due to moisture absorption.
Furthermore, an expanded molded article derived from starch such as (1) and (2) mentioned above, has a porous matrix structure of starch formed through steam expansion which is a starch structure having a very wide surface area thereon. Since an expanded molded article is expanded and foamed by using evaporative expansion of water, a surface of the starch structure has strong hydrophilicity. On this account, the starch structure is an important factor showing excellent biodegradability. At the same time, it absorbs water and moisture very easily.
Even if the water-resistant coating such as (3) and (4) is formed to protect the molded article which easily absorbs water and moisture, it is possible to prevent water in liquid form from penetrating into the starch structure, while it is impossible to block water vapor in gaseous form.
Also, even if an expanded molded article derived from starch is covered with a film or coating made from a biodegradable plastic available at present, it is difficult to block water vapor as the above case unless it is more than hundreds of μm thick.
Therefore, an expanded molded article derived from starch (starch structure) with high moisture resistance is especially desired.